PLOS ONE Digging the New York City Skyline Soil Fungal Communities in Green Roofs and City Parks

Affiliation: Department of Biology, Barnard College of Columbia University, New York, New York, United States of America

Sara G. Payne,

Abstract

In urban environments, green roofs provide a number of benefits, including decreased urban heat island effects and reduced energy costs for buildings. However, little research has been done on the non-plant biota associated with green roofs, which likely affect their functionality. For the current study, we evaluated whether or not green roofs planted with two native plant communities in New York City functioned as habitats for soil fungal communities, and compared fungal communities in green roof growing media to soil microbial composition in five city parks, including Central Park and the High Line. Ten replicate roofs were sampled one year after planting; three of these roofs were more intensively sampled and compared to nearby city parks. Using Illumina sequencing of the fungal ITS region we found that green roofs supported a diverse fungal community, with numerous taxa belonging to fungal groups capable of surviving in disturbed and polluted habitats. Across roofs, there was significant biogeographical clustering of fungal communities, indicating that community assembly of roof microbes across the greater New York City area is locally variable. Green roof fungal communities were compositionally distinct from city parks and only 54% of the green roof taxa were also found in the park soils. Phospholipid fatty acid analysis revealed that park soils had greater microbial biomass and higher bacterial to fungal ratios than green roof substrates. City park soils were also more enriched with heavy metals, had lower pH, and lower quantities of total bases (Ca, K, and Mg) compared to green roof substrates. While fungal communities were compositionally distinct across green roofs, they did not differentiate by plant community. Together, these results suggest that fungi living in the growing medium of green roofs may be an underestimated component of these biotic systems functioning to support some of the valued ecological services of green roofs.

Funding: This project was funded by the Barnard College Presidential Research Award. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Introduction

Green roofs have become increasingly popular in urban sustainability initiatives, as they provide a number of ecosystem services that mitigate the effects of urbanization such as decreased storm water runoff, enhanced building energy-use efficiency, and reduced urban heat island effects [1] –[3]. An additional benefit of green roofs that has not been fully explored is the potential reservoir of habitats for biota residing in or migrating across the city [4]. Like city parks and other urban green spaces, green roofs provide vegetated islands that birds, insects, and other airborne organisms may make use of in the urban matrix [5] –[8]. However, the historical focus of green roof research has been on infrastructure and engineering, so the role of green roofs as biodiversity reservoirs has only recently been emphasized [5]. Understanding how biodiversity is assembled and maintained will be useful for managing green roof systems to maximize their provision of ecosystem services while simultaneously minimizing external inputs and roof maintenance. In addition to their practical aspects, green roofs can also function as ideal experimental systems for asking ecological questions about community assembly and habitat fragmentation.

The community composition of the vegetation planted on green roofs may have a major impact on their associated biodiversity. Most green roofs in North America are planted with non-native species of Sedum (Crassulaceae), which are succulent, perennial ground plants tolerant to the extreme conditions found on rooftops [9]. [10]. However, if one of the aims of installing a green roof is to maintain local biodiversity, then non-native Sedum may not be the optimal vegetation choice. One of the challenges of installing native plants on green roofs is that they must be able to withstand the harsh rooftop environments and demonstrate performance that equals or surpasses monocultures of Sedum in terms of the additional ecosystem services they provide. A recent study in Nova Scotia found that mixtures of Sedum and different life forms of native plants, such as grasses and forbs, displayed optimum performance in terms of building temperature reduction and water capture [11]. However the associated non-plant biodiversity of these communities was not assessed. Positive links between biodiversity and ecosystem function have long been recognized in plant community ecology, but since engineers and architects have led most green roof initiatives [12]. attention to the particular plant community installed on the roofs has not been a focus. In fact, a recent review of the green roof literature identified only five studies that had specifically manipulated plant diversity in green roof communities [13]. From an ecological perspective, if green roofs are to function as effective biodiversity reservoirs, then the particular assemblage of plants on the roof may have a major impact on which non-plant taxa are attracted to and can utilize the habitat in different regions.

Fungi residing in the growing media are one of the integral components of the green roof biota that may influence the functionality and longevity of green roofs. Analogous to soil, the roof substrate likely contains a diverse array of microorganisms that help sustain the roof vegetation. While the physicochemical composition of green roof media has received considerable attention [14]. fungal diversity and function on green roofs have not yet been examined despite their integral function in nutrient cycling, symbiosis, and plant productivity. The assemblage of microbes in roof growing media likely depends on numerous biotic and abiotic factors such as the plant community, initial substrate inoculum, local climate, moisture availability, and airborne inoculum [15]. [16]. Bacteria and fungi are ubiquitous in ecosystems and due to their small sizes, many can readily disperse through the air [17] –[19]. As such, the distance that green roofs are from larger, intact vegetation patches such as city parks and forest fragments may be important in determining the numbers and types of microbial taxa able to disperse to a given roof. The initial fungal inoculum may also exhibit strong priority effects preventing new fungal species from establishing. Priority effects have been demonstrated for wood decomposer fungi [20]. [21]. ectomycorrhizal fungi [22]. and yeasts [23]. so it is plausible that historical contingencies from starting inoculum strongly influence green roof fungal composition.

This study aimed to address the following questions related to fungal communities in green roof substrates and nearby city park soils: 1) Do green roofs in New York City function as biodiversity reservoirs for fungi?; 2) Is there evidence for spatial structuring of green roof fungal communities across New York City? 3) Does vegetation type influence community composition of green roof fungi?; 4) How much overlap is there in fungal community composition of the green roof substrates and city park soils?